1. Technical Field
The present invention relates to a technique of applying a predetermined voltage waveform to an electric load having a capacity component and thus driving the electric load.
2. Related Art
Various types of electric loads driven by the application of a voltage are known and there are a number of electric loads having a capacity component such as a so-called piezoelectric element and a liquid crystal screen. In an electric load having a capacity component, the applied voltage rises as electric charges are supplied to the load, whereas the applied voltage decreases as electric charges are discharged from the load. Therefore, if a capacitor is used when driving a load having a capacity component, the load can be efficiently driven. That is, in the case of lowering the applied voltage, electric charges stored in the load are collected and stored in the capacitor. Then, at the time of raising the applied voltage, the electric charges stored in the capacitor are supplied to the load to raise the applied voltage. In this manner, the applied voltage to the load can be raised without supplying power to the load from a power source.
Of course, if the applied voltage to the load exceeds the terminal voltage of the capacitor, electric charges cannot be supplied from the capacitor. Therefore, the applied voltage to the load cannot be raised to the terminal voltage of the capacitor or higher. Thus, a technique of efficiently driving a load having a capacity component with as little power supply as possible from a power source is proposed in which plural capacitors having different terminal voltages are provided and then capacitors to connect to the load are switched one after another, as disclosed in JP-A-2003-285441.
In the proposed technique, a power source and plural capacitors are connected to each other and each capacitor is charged in advance so that the capacitors have different terminal voltages from each other. Then, in the case of raising the applied voltage to the load, capacitors to connect to the load are switched from a capacitor having a low terminal voltage to a capacitor having a high terminal voltage. Thus, the applied voltage to the load can be raised without supply of power from the power source. On the other hand, in the case of lowering the applied voltage to the load, a capacitor having a terminal voltage that is slightly lower than the applied voltage is connected to the load and electric charges accumulated in the load are shifted to the capacitor. Thus, the applied voltage to the load is lowered.
As the applied voltage is consequently lowered to the terminal voltage of the capacitor, the capacitor to connect to the load is switched to a capacitor having a slightly lower terminal voltage. As the capacitors are switched one after another in this manner and electric charges of the load are shifted to the capacitors, the applied voltage can be lowered. After that, in the case of raising the applied voltage again, by utilizing the electric charges thus stored in the capacitors, it is possible to efficiently drive the load having the capacity component without supplying power from the power source.
However, with the proposed technique, there are cases where the terminal voltage of a capacitor gradually rises while a voltage is applied to drive the load, making proper driving of the load difficult. For example, it is now assumed that the applied voltage is to be raised during the course of lowering the applied voltage by connecting a certain capacitor to the load and collecting electric charges. In this case, in order to raise the voltage to apply to the load, the capacitor is switched to a capacitor having a higher terminal voltage (or power source). Therefore, electric charges are one-sidedly accumulated in the capacitor connected to the load at the time of lowering the applied voltage. As this one-sided accumulation is repeated, the quantity of electric charges in the capacitor is increased and the terminal voltage rises accordingly. In this manner, depending on the waveform of a voltage applied to the load, the quantity of electric charges accumulated in the capacitor exceeds the quantity of electric charges discharged from the capacitor. Consequently, the terminal voltage of the capacitor may rise.